This invention relates to a method of trimming the resistance value of a polycrystalline silicon resistor, more particularly a method of trimming by electrical means the value of a polycrystalline silicon resistor utilized as the resistance element of a semiconductor integrated circuit.
The resistance value of a resistance element incorporated into an integrated citcuit generally deviates from the designed value, due to variation in the manufacturing condition of the resistance element. To achieve the accuracy required beyond the limits of manufacturing condition, it is common practice to use one of the two trimming methods. The first method is laser trimming of a thin film metal resistor, disclosed in a paper "An Integrated Circuit 12 Bit D/A Converter" by R. B. Craven, ISSCC Digest of Technical Papers, pages 40-41, 1975 and a paper "A High Yield Second Generation 10 Bit Monolithic DAC" by P. Holloway et al, ISSCC Digest of Technical Papers, pages 106-107, 1976. The second method is selective short-circuiting of zener diodes by a large current to produce correctional circuits, disclosed in a paper "A Precision Trim Technique for Monolithic Analogue Circuits" by G. Erdi, ISSCC Digest of Technical Papers, pages 192-193, 1975, and a paper "A Monolithic 12 Bit D/A Counter" by D. T. Comer, ISSCC, Digest of Technical Papers, pages 104-105, 1977.
However, the method of trimming the thin metal film resistor by a laser beam introduces stress caused by local heating of the resistor, thus causing instability. Moreover, as the resistance value of the resistance element is trimmed before it is mounted on the package, the resistance value may be caused to vary by the heat treatment of the package. Furthermore, it is necessary for the laser trimming to use an expensive trimming equipment. On the other hand, the method of selective short-circuiting of zener diodes by large current cannot correct the resistance value directly, so that complicated and space consuming additional correctional circuits are required, resulting in a low yield of the product. In typical semiconductor integrated circuits available on the market, the area of the adjusting circuit is so large that about one half the area of the chip is occupied by the adjusting circuit.
Accordingly, it has been desired to establish a method of trimming the resistance value of a resistance element to be incorporated into an integrated circuit so as to readily correct the resistance value to within desired limits after fabrication of the resistor element in the integrated circuit. The provision of a simple and precise trimming method suitable for polycrystalline silicon resistors is highly desirable and valuable because polycrystalline silicon resistors are quite suitable for use in fabricating monolithic integrated circuits.